1. Field of the Invention
The present invention relates to a two-dimensional image display apparatus in which a laser beam is irradiated onto the display surface such as a screen or the like while the laser beam is deflected by deflecting means such as a galvanometer or the like so that an image is displayed in accordance with input data.
2. Description of Background Information
An apparatus has already come to be widely known as Laser display apparatus, in which a laser beam emitted from a light source as means for radiating a laser beam is irradiated onto the screen while the laser beam is deflected in horizontal and vertical directions in accordance with input data carrying two-dimensional coordinates.
FIG. 1 shows a conventional example of such a display apparatus. In the conventional apparatus shown in the figure, a data generating circuit (not shown) such as a microcomputer or the like previously stores coordinates (x.sub.i, y.sub.i) of each of designated points in a group of display points corresponding to an image to be displayed on the display screen. Input data of a digital signal carrying the coordinates (x.sub.i, y.sub.i) of the designated points is supplied to a drive signal generating device 1 at predetermined timings. In the drive signal generating device 1, the input data is converted into an analog signal by means of a D/A converter 11. The analog signal is supplied to a drive circuit 12 wherein power amplification of the input analog signal is performed. A driving voltage V.sub.x which is proportional to the horizontal coordinate x.sub.i of the designated point is applied from the drive circuit 12 to a driving motor 2a of a galvanometer 2. A rotating mirror 2b is directly coupled to the drive motor 2a and a rotational angle is determined in accordance with the driving voltage V.sub.x. Likewise, a driving voltage V.sub.y which is proportional to a vertical coordinate y.sub.i of the designated point is applied from the drive circuit 12 to a driving motor 3a of a galvanometer 3 and a rotational angle of a rotating mirror 3b coupled directly to the driving motor 3a is determined in accordance with the driving voltage V.sub.y. In this way, the galvanometers 2 and 3 function as deflecting means for deflecting a laser beam in the horizontal and vertical directions.
A laser beam 5 radiated from a light source 4 is reflected by the rotating mirrors 2b and 3b and is irradiated onto a screen 6 as a display surface. The laser beam 5 is scanned on the screen 6 in the direction indicated by the arrow x, namely, horizontal direction in accordance with the rotation of the rotating mirror 2b. The laser beam 5 is similarly scanned on the screen 6 in the direction indicated by the arrow y, namely, vertical direction in accordance with the rotation of the rotating mirror 3b. As mentioned above, a light spot of the laser beam 5 drawn on the screen, that is, a display point is determined according to the coordinates (x.sub.i, y.sub.i) of the designated point of input data.
In FIG. 2, the laser beam is deflected in the horizontal direction at an angle .theta..sub.x by means of the rotating mirrors 2b and 3b operating as a center point O in proportion to the horizontal coordinate x.sub.i, and is irradiated to a point P on the screen 6. Although a length L.multidot..theta..sub.x of an arc Q'P' on a spherical surface 6' of a radius L is proportional to x.sub.i, a displacement QP in the direction x on the screen 6 is not proportional to x.sub.i. Also with respect to the vertical direction, the displacement in the direction y is not proportional to y.sub.i. As described above, the coordinates (x.sub.i, y.sub.i) of the designated point do not coincide with coordinates (x.sub.j, y.sub.j) of a display point on the screen since the screen 6 does not have a spherical surface. For instance, in the case where the input data carries coordinates of the designated points in lattice-shape as shown in FIG. 7A, the display points drawn on the screen 6 are distributed as shown in FIG. 7B. Thus, there has been a problem of what is called a pincushion distortion occurs.